The present invention relates to a light control element to be used in combination with an edge-light-type backlight unit, a surface light source device, and a liquid crystal display device, for example.
In recent years, liquid crystal display devices have been widely been used as display portion of fixed-type apparatuses having relatively large-sized display screens as typified by monitors of home-use, large-sized television set and desktop personal computer, and for display portion of portable devices as typified by notebook-sized personal computer, mobile phone, digital still camera (DSC) and digital video camera (DVC).
The liquid crystal display device is basically made of a backlight unit (surface light source device) and a liquid crystal display panel. The backlight unit includes those of direct type having a light source disposed right under the liquid crystal display panel, and those of edge light type using a light guide plate, the light guide plate being disposed such that a light source faces one of side edge portions of the light guide plate. The edge light type is widely adopted to portable devices in order to reduce device size.
Methods of display on the display portion of portable devices may be classified into two types. One of which is a method of placing the first priority on the front luminance, while accepting lowering in visibility for wide angle viewing due to insufficient energy of light. Another is a method of pursuing uniform visibility for wide angle viewing, by widening the viewing angle while sacrificing the front luminance to some degree. The former is adopted to devices intended for personal use, as typified by notebook-sized personal computer and mobile phone, whereas the latter is adopted to devices, as typified by DSC and DVC, intended for use in a case where captured images are viewed by many people at different viewing angles.
As a method of directing light from the backlight towards the liquid crystal display panel in the edge light system, there has been known a method of using a light control element having a structured surface such as triangular prisms, and disposing the light control element adjacent to the light guide plate, while directing the structured surface opposed to the plane of output side of the light guide plate (see Japanese Patent Application Publication No. Hei 6-18879, for example).
On the other hand, Japanese Patent Application Publication No. Hei 7-325208 listed below discloses a lens sheet disposed adjacent to the light guide plate on the plane of output side thereof. The lens sheet has prism-like structures that face the light guide plate. In the shape of each prism, the oblique surface thereof is bent in the cross-sectional profile, so as to make prism angle of the base larger than the apex angle.
As described above, for the case where the structured surface is formed with a triangular prism form, light output from the light guide plate can be deflected towards a desired direction by examining angle of oblique surface of prism and so forth. However, the triangular prism is only to direct the light, output from the light guide plate within a small range of directivity, towards the liquid crystal display panel while keeping the angle unchanged, so that the range of output angle of light output from the prism sheet, that is, output viewing angle cannot be controlled by the prism portion. This is only permissible for portable devices for personal use in the above description, but not suitable for wide angle viewing by many people.
On the other hand, the lens sheet described in Patent Document 2 expands the range of output angle of light, by bending the oblique surface of prism in the cross-sectional profile, so as to make the prism angle at the base larger than the apex angle. However, direction of reflection of light discontinuously changes as being bounded by the bent portion of the prism surface, so that intensity distribution of the output light tends to cause non-uniformity, inevitably degrading quality of image on the liquid crystal display portion.